Dental restoratives are materials that restore both the appearance and function of a defective tooth structure. Features desirable in a dental restorative are good coloring to match teeth, high polishability, limited or no shrinkage upon curing, good mechanical properties, sustained fluoride release, radiopacity, excellent bonding with the natural tooth structure, and a wear resistance similar to enamel. Examples of dental restoratives are glass ionomer restoratives, composite resin restoratives, and composite resin/glass ionomer hybrid type restoratives.
Glass ionomer restoratives were introduced to dentistry as dental luting cements and restorative materials about two decades ago. The material is packaged as a liquid/powder two-part system that must be mixed immediately prior to application. Hardening of a glass ionomer restorative is achieved through the reaction of a poly(carboxylic acid) with an ion-leachable fluoroaluminosilicate glass in the presence of water, forming a crosslinked network structure. Water is an essential ingredient needed for the setting reaction of a glass ionomer material.
Glass ionomer restoratives have the desirable properties of good adhesion to unconditioned dentin, cariostatic effect due to long-term fluoride release, and excellent biocompatibility. However, some undesirable properties that have limited their acceptance are their poor handling properties, lack of a command cure, moisture sensitivity during the early stage of the setting reaction, poor mechanical properties, and poor wear resistance. Glass ionomer restoratives find use in areas of the oral cavity where low stress is encountered as underfilling materials, luting cements, and in some cases as filling materials for gingival lesions.
Composite resin restoratives were created in parallel with glass ionomer restoratives. Composite resin restoratives appeared to solve all the problems associated with glass ionomer restoratives. Composite resin restoratives offer excellent aesthetics, easy handling, and much improved mechanical strength and wear resistance. However, composite resin restoratives alone do not release fluoride. Composite resin restoratives are comprised of (meth)acrylate resins, defined as resins containing either acrylate or methacrylate groups, reinforced with an inert inorganic filler. Hardening of the composite resin is achieved through free-radical polymerization of the (meth)acrylate monomers using a photoinitiator, a heat-cure initiator, or a redox initiator system.
Attempts have been made to combine glass ionomer and composite resin chemistries by introducing polymerizable (meth)acrylate monomers/oligomers/prepolymers and free-radical initiators into the glass ionomer system, leading to the development of a hybrid material. Two types of hybrid materials were created: a "resin-modified glass ionomer" (RMGI) type and a "compomer" type.
RMGI materials retain most of the characteristics of a conventional glass ionomer in that water is an essential ingredient, they are packaged as a liquid/powder two-part system that must be mixed prior to use, and there is a significant contribution from the ionic acid-base setting reaction. RMGI materials have improved the mechanical properties, and have eliminated the moisture sensitivity, of the conventional glass ionomer materials. However, their application is still limited to use in low stress bearing areas of the mouth due to their inadequate mechanical strength and wear resistance. Another problem is that RMGI restoratives, once placed in a restoration, tend to absorb excessive and uncontrolled amounts of water, causing crowns cemented with RMGI materials to fracture due to excessive hygroscopic expansion of the materials.
With RMGI materials, three curing mechanisms are possible: an acid-base setting reaction of the conventional glass ionomer type, photo-curing using photo-initiators, and chemical-curing using redox initiators via free-radical polymerization of the polymerizable vinyl groups.
The second type of hybrid material, called "compomers," are light-curable single-part paste systems. They contain the following key ingredients: a reactive fluoroaluminosilicate glass, (meth)acrylate monomers containing acid functional groups, and a curing initiator, usually a photoinitiator. They may also contain other copolymerizable (meth)acrylate monomers which do not contain acid functional groups. Water is absent from the composition; the material is in an anhydrous (non-aqueous) form.
The primary setting reaction of a compomer is free-radical photo-polymerization involving the vinyl functional groups. Compomers, as single-part resin-based materials, can only be hardened clinically through photo-curing. In this respect, compomers are clinically used in a way identical to light-curable composite resins. Light-curing of the (meth)acrylate monomers results in an inter-connected polymer network which is reinforced by enclosed reactive filler particles. This provides the immediate strength and resistance needed in the oral cavity. At this stage, existing acidic groups on the polymers remain inactive since the compomer is an anhydrous form which prevents the acid-base ionic reaction from occurring. Once placed into the moist environment of the mouth, however, the compomer begins to absorb water, thus completing the ingredients necessary to initiate an ionic acid-base reaction, i.e., acidic groups, a reactive filler, and water. This secondary ionic acid-base reaction serves to release fluoride, although it contributes little to the integrity and mechanical properties of the cured compomer. Besides long-term release of fluoride, another desirable characteristics of a compomer is its ability to absorb a small amount of water and, as a result, expand slightly to alleviate the shrinkage stress caused by the polymerization of the (meth)acrylate resin, thus contributing to good marginal integrity.
Compomers are composite resin/glass ionomer hybrid restoratives that combine, in one material, the desirable properties of glass ionomer restoratives and composite resin restoratives. Compomers have good aesthetics, easy handling, a command cure mechanism, good mechanical properties such as strength and wear resistance, long-term fluoride release, and a small amount of hygroscopic expansion. As a result, long-lasting restorations can be obtained with compomer materials, and their applications are being extended to areas in the oral cavity subject to moderate to high stresses. However, improvements in mechanical strength and wear resistance are still needed before compomers can truly replace composite resins as an anterior/posterior restorative material.